Sitting on the shoulder of the constellation of Orion with a distinctly reddish hue, Betelgeuse is one of the more recognizable stars in the night sky. But it is also one of the fastest changing. While Ptolemy described it orange-tawny, Chinese observers three centuries earlier say it was yellow.
More recently, astronomers classified Betelgeuse as a red giant, a star that is near the end of its life. That meant Betelgeuse is due to explode as a supernova sometime in the next few hundred thousand or a million years. They also discovered it was pulsating — becoming brighter and dimmer over periods ranging from a few months to a few years.
Then things started happening quickly. In 2019, Betelgeuse dimmed suddenly before returning to normal a few months later as curious astronomers watched from afar. The question on all lips was whether Betelgeuse was about to explode. In the event, astronomers concluded that the dimming was probably the result of a dust cloud and that Betelgeuse’s supernova was probably tens or hundreds of thousands of years away.
Now Hideyuki Saio at Tohuko University in Japan and colleagues have reanalyzed the data and say Betelgeuse may be closer to blowing than anyone thought. “We conclude that Betelgeuse is in the late stage of core carbon burning, and a good candidate for the next Galactic supernova,” they say.
First some background. The way stars change throughout their lives is a well-studied process in astronomy. In general, stars form as balls of hydrogen and helium gas which collapse, causing hydrogen nuclei to fuse releasing huge amounts of energy. This energy heats the star preventing further collapse but only as long as the star has the hydrogen fuel to support it.
Over time, however, the hydrogen runs out and the star begins to fuse helium. This causes the star to expand, become redder and eventually to burn heavier elements like carbon forming an iron core.
Iron is the most stable element and so cannot fuse to release energy. So when the carbon runs out, the star collapses in a supernova event to form a neutron star or, if it is massive enough, a black hole.
Astronomers have long known that Betelgeuse is a supergiant red star at the end of this sequence and that a supernova is likely. The big question is when.
Now Saio and co say a big clue comes from the way the star pulsates. The pulsations are essentially vibrations in which a pressure wave causes the star to burn more and becomes temporarily bigger and brighter, which reduces the burning rate causing it to dim again.
These pulsations can be complex but accurate measurements of them place important limits on the star’s mass and the type of fusion processes going on inside.
Saio and co created a model that simulated Betelgeuse’s pulsations and say it provides unique insight into the star’s internal status. These pulsations occur over 2200 days, 420, 230 and 185 days. The team also simulated the light that such a star would produce and say it exactly matches that of Betelgeuse, which must have a radius of about 1300 times the radius of our Sun.
The only way for Betelgeuse to pulsate in this way is if it is at the very end of its carbon burning period. That means the collapse of this star’s core and the accompanying supernova is imminent. “After carbon is exhausted in the core, a core-collapse leading to a supernova explosion is expected in a few tens of years,” they conclude.
If it does blow, Betelgeuse will become the brightest star in the night sky and one of the most fascinating and spectacular phenomena in human history. However, astronomers believe it is far enough away not to threaten life on Earth.
Now Betelgeuse has begun to behave oddly again, brightening by 50 per cent in the last month or so. Nobody knows whether this is the immediate precursor to a supernova but either way, astronomers are studying this mysterious star with interest.
Ref: The evolutionary stage of Betelgeuse inferred from its pulsation periods : arxiv.org/abs/2306.00287